I play guitar so that helps me understand what you are getting at. You've got some sort of voltage-controlled analog effect box circuit that you'd like to put under digital Arduino control by using PWM and an R/C filter to generate the control voltage for the effect. I've thought of that before, and it would seem to be fine in theory (if your circuit is happy with 0-5V for your analog control, and doesn't take too much current); otherwise as mentioned you'll want an op amp to get a different range.

Yes, certainly that is a viable application. And even the 0-5 vdc restriction is easy to overcome with simple op-amp with the gain set to whatever DC voltage range desired after the R/C filtering is done.

It's a simple oscillator circuit I have to re-dig-up. It used a resistor and capacitor and a pot to control the tone, could have added volume control. I could replace the pot with a transistor couldn't I? As a valve?

Finished reading the Simple filter piece.

Increase in C lowers both Xc and F, Increase in F lowers Xc and RC

And I think that making Xc small compared to R is desired.So that lets low frequencies pass hence the moniker.

Suppose I want to change the duty cycle of the PWM from 10% to 90% smoothly in 1/100th or 1/1000th of a second? Would this filter have trouble following that? Again: the audio frequency is generated on a different circuit, *not* the PWM frequency being converted to analog DC to control a transistor.

I have an AC oscillator making the wave. Not using PWM frequency for the wave. Wanting to use PWM to affect the external sound circuit in two ways, maybe find a third.

It's like being able to control the slide of a trombone when something -else- vibrates the air.I'm trying to control the slide with PWM, not vibrate the air, move the bow not be the string.And I wonder how fast I'll be able to move the bow smoothly.